The verification of the existence of a third form of carbon termed “fullerenes” in 1990 touched off an intense wave of research and development aimed at maximizing the potential of this “new” material. The term “fullerene” is often used to designate a family of carbon molecules, which have a cage-like hollow lattice structure. These “cages” may be different forms, such as spheres (“buckyballs”), or tubes (“nanotubes”). See Robert F. Curl and Richard E. Smalley, Fullerenes, Scientific American, pg 54-83, October 1991 (describing the properties and structure of fullerenes).
2.1. Carbon Nanotubes
Carbon nanotubes can exist as closed concentric multi-layered shells or multi-walled nanotubes (MWNTs) or as a single-walled nanotubes (SWNTs). However, the preferred carbon nanotube for industrial application is a single-wall carbon nanotube.
Carbon nanotubes, and in particular single-wall carbon nanotubes, because of their wide-range of electrical properties are used for making electrical connectors in micro devices such as integrated circuits or in semiconductor chips used in computers because of their electrical conductivity and small size. Carbon nanotubes are also used as antennas at optical frequencies, and as probes for scanning probe microscopy such as are used in scanning tunneling microscopes (STM) and atomic force microscopes (AFM).
In addition, because of their mechanical strength, carbon nanotubes are also used as strengthening agents in any composite material in conjunction with carbon black in tires for motor vehicles or in conjunction with graphite fibers in airplane wings and shafts for golf clubs and fishing rods.
Carbon nanotubes may also be used in combination with moldable polymers that can be formed into shapes, sheets or films to make electrically conductive shapes, sheets or films and are also useful as supports for catalysts used in industrial and chemical processes such as hydrogenation, reforming and cracking catalysts. Thus, in view of their broad range of applications, a convenient easily manipulable form of carbon nanotubes would be extremely useful.
Both MWNTs and SWNTs have been produced and the specific capacity of these materials has been evaluated by vapor-transport reactions. For example, modification of the spark erosion technique enabled the preparation of macroscopic quantities of nanotubes, which were then evaluated using vapor-transport reactions and X-ray diffraction. See for example, Zhou et al., Defects in Carbon Nanotubes, Science: 263, 1744-47, 1994. However, it is believed that single-walled carbon nanotubes hold the most promise for future nanotube based materials.
2.2. Single Walled Carbon Nanotubes
Since their discovery in 1991, single-walled nanotubes of carbon have been extensively investigated. See Dresselhaus et al., Science of Fullerenes and Carbon Nanotubes, Academic Press, (1996) (comprehensive and cumulative review of the state of the art in 1996). Such studies included, inter alia, scanning tunnel spectroscopy studies, transport measurements and magnetoresistance studies. See Dresselhaus et al., Science of Fullerenes and Carbon Nanotubes, Academic Press, 756-869 (1996). Based on scanning-tunneling microscopy (STM) images and electron diffraction studies, single-walled nanotubes (“SWNTs”) were shown to consist of a seamless cylinder of a graphitic sheet capped by hemispherical ends composed of pentagons and hexagons. See Ge et al., Appl. Phys. Lett. 65 (18), 2284-2286 (1994). See also Sattler, Carbon 33(7), 915-920 (1995). In this cylindrical shape, the graphite is in either zigzag or arm chair helical configuration. See Ge et al., Appl. Phys. Lett. 65 (18), 2284-2286 (1994). Such cylinders had a diameter of about 1 nm. See Ge et al. Appl. Phys. Lett. 65 (18), 2284-2286 (1994). Curves observed in high-resolution transmission electron microscope (HRTEM) images of SWNTs indicate that the single-walled tubes are more pliable than their multi-walled counterparts.
Single-walled carbon nanotubes form the basis of materials with exceptional mechanical and electrochemical properties, including polymer reinforcement and molecular electronics. Despite their intrinsic rigidity and high anisotropy, the current available macroscopic forms of SWNTs are isotropic and rather fragile. Vigolo et al., Macroscopic Fibers and Ribbons of Oriented Carbon Nanotubes, Science, 290, 17, 1331.
Previous work on carbon nanotubes (both single-walled and multi-walled), has been carried out on intractable forms of this material. Yakobson et al., Fullerene Nanotubes: C1,000,000 and Beyond, American Scientist, 1997, 85, 324-337. This form of the material is not amenable to many of the processing steps that are necessary if the single-walled carbon nanotubes (SWNTs) are to reach their full potential, particularly in applications that require these materials in the form of polymers, copolymers, composites, ceramics and moldable forms.
Currently, the carbon nanotube raw material is produced in bulk as a fluffy solid. As they form in the gas phase, the carbon nanotubes condense into a solid and naturally aggregate with one another to forms ropes of nanotubes. These ropes further agglomerate to form larger random tangles. This tangled form of the bulk material cannot be used in many of the projected applications. Additionally, the “as made” nanotube material's do not exhibit the conductivity, strength, thermal properties, surface area or electronic nature of the carbon nanotube molecule itself.
While present forms of the SWNTs can be heterogeneously dispersed in various media, in most cases the interactions between the SWNTs and the media and between the SWNTs themselves are simply physical, and without the formation of chemical bonds. Thus, without further manipulation (either chemical or physical) the advantageous properties of the SWNTs are unlikely to be realized on a macroscopic level.
Carbon nanotubes, and more specifically, single-walled carbon nanotubes are completely intractable solids, in that they are not soluble in any liquid and as a result are very difficult to manipulate. In order to make a fiber, film or coating from a solid material either dissolved or suspended in a liquid, a concentration of at least 1% by weight of the material is desirable due to limitations in viscosity and mass transfer (“Fundamentals of Fibre Formation”, A. Ziabicki, John Wiley and Sons (1976)). Preferentially, the carbon nanotubes are individually suspended in a liquid at these or comparable concentrations to form a fiber, film or coating.
Solubilization of single-walled carbon nanotubes have been achieved by various techniques including the addition of surfactants or functionalization of the end caps and side-walls of the nanotubes. However, each of these methods has inherent deficiencies, such as low concentration of nanotubes or modification of the intrinsic carbon nanotube's properties. Concentrations of less than 1% by weight of carbon nanotubes have been achieved; for example, Hirsch et al. were able to functionalize single-walled nanotubes with large organic molecules making them soluble in highly polar solvents (tri-chloromethane) at concentrations of 0.5% wt/wt (50 mg/mL). Hirsch et al., J. Am. Chem. Soc, 124, 760-761 (2002). Specifically, Hirsch et al. observed that functionalized single-walled carbon nanotubes were very soluble in CHCl3, CH2Cl2, acetone, methanol, ethanol and water. Hirsch et al., J. Am. Chem. Soc, 124, 760-761 (2002). Although lesser concentrations may be used for incorporation into some composite systems, most applications and systems preferentially require higher concentrations of suspended material. Ideally, the carbon nanotubes should be preferentially monodispersed (i.e., highly separated) in the suspension for the development of many of the projected applications.
There is a report of nanotubes suspended in a surfactant mixture, where the surfactant was sodium dodecylsulfate (SDS) (see B. Vigolo, et al., Science 290, 1331 (2000)). SDS which has a Critical Micelle Concentration (CMC) of about 8 mM or ten times greater than cetyl trimethyl ammonium bromide (CTAB) The maximum solubility of NT was reported to be about 3 g/L.
Additionally, in many applications, toxic solvents do not lend themselves to industrial processing and can add to cost of production. While chemically attaching molecules to the nanotubes may increase their solubility, it also alters their electronic and mechanical properties and the attached moieties cannot be easily removed once the nanotubes are incorporated into a host system (such as a paint or plastic or even a bulk form of the nanotubes).
Well-dispersed, non-aggregating, highly concentrated, suspended forms of carbon nanotubes and methods of producing the same are necessary to advance the different technologies. Although long believed to be impossible, the present invention teaches such a procedure for the dispersion and suspension of carbon nanotubes, and specifically, single-walled nanotubes. The present invention teaches the use of surfactants to produce highly concentrated compositions of suspended carbon nanotubes (to concentrations of greater than 1 wt %).